Coating compositions, e.g., liquid and powder coating compositions, are used in a wide variety of applications, including, for example, the automotive, appliance and industrial markets. Coatings are often used to provide decorative qualities and/or corrosion protection to the substrates over which they are applied. Correspondingly, applied coatings are typically required to have at least a continuous defect free surface, and in the case of decorative finishes, also a very smooth surface. The automotive industry has particularly strict requirements as to the smoothness of the coatings that are used, as is the case with automotive clear top-coat compositions.
Coating compositions typically contain a flow control agent (also referred to as a flow modifier) to improve the appearance of the cured coating. Flow control agents have surface active properties and are thought to improve the appearance of a cured coating by altering the flow and leveling of the applied coating during its cure cycle. Flow control agents containing functional groups, e.g., carboxylic acid groups and/or hydroxyl groups, are known, and in addition to enhancing appearance, can also improve the adhesion of the coating to the substrate over which it is applied, and/or improve the adhesion or compatibility of a subsequently applied coating.
Coating compositions are typically required to provide optimum properties, e.g., appearance and/or corrosion resistance, at a minimum film thickness. For example, in the automotive industry, clear top-coats are typically required to have cured film thicknesses of no greater than 50 microns (2 mils). Advantages associated with coatings applied at lower film thickness include, for example, reduced material costs and weight gain of the coated ware, which is particularly desirable in the aircraft industry. However, as the film build of an applied coating composition is decreased, the appearance of the resulting cured coating typically diminishes, for example, as evidenced by lower measured gloss values.
In addition to the application of coatings at lower film builds, investigation and development in recent years has been directed towards reducing the environmental impact of coatings compositions, in particular that associated with emissions into the air of volatile organics during their use. Accordingly, interest in coatings having lower volatile organic content (VOC), e.g., powder coatings and high solids coatings, has been increasing. Powder coating compositions are free flowing particulate compositions that are substantially free of solvents. The appearance of powder coatings typically degrades rather precipitously with decreasing film thickness, e.g., at film thicknesses less than 75 microns (3 mils), and in particular at film thicknesses less than 50 microns (2 mils). In the absence of solvents that can enhance the flow and leveling of an applied coating, a flow control agent is a critical component in the majority of powder coating compositions.
It would be desirable to develop coating compositions, e.g., powder coating compositions, that have improved properties, such as appearance. In particular, it would be desirable to develop coating compositions that have improved properties, such as appearance, at lower film thicknesses, e.g., film thicknesses less than or equal to 50 microns.
U.S. Pat. No. 5,212,245 describes thermosetting powder coating compositions comprising a curable particulate resinous material and a flow control agent. The flow control agent of the '245 patent is described as a copolymer of an alkyl acrylate and/or alkyl methacrylate containing from 6 to 20 carbon atoms in the alkyl group, and a hydroxyalkyl acrylate and/or hydroxyalkyl methacrylate. The flow control agents of the '245 patent are not described as being either block copolymers or prepared by controlled or living radical polymerization methods.
International Patent Publication No. WO 97/30131 describes curable coating compositions comprising either a liquid or particulate curable film-forming resinous material and a flow control agent. The flow control agent of Publication No. WO 97/30131 is described as being a copolymer of at least one alkyl acrylate and/or alkyl methacrylate containing from 1 to 20 carbon atoms in the alkyl group, an amino functional acrylate and/or amino functional methacrylate, and optionally a hydroxyalkyl acrylate and/or hydroxyalkyl methacrylate. The flow control agents of Publication No. WO 97/30131 are described as being prepared by non-living free radical polymerization methods.
U.S. Pat. Nos. 5,807,937, 5,789,487 and 5,763,548, and International Patent Publication No.'s WO 98/40415, WO 98/01480, WO 97/18247 and WO 96/30421 describe a radical polymerization process referred to as atom transfer radical polymerization (ATRP). The ATRP process is described as being a living radical polymerization that results in the formation of (co)polymers having predictable molecular weight and molecular weight distribution. The ATRP process is also described as providing highly uniform products having controlled structure (i.e., controllable topology, composition, etc.). The '937 patent also describes (co)polymers prepared by ATRP, which are useful in a wide variety of applications including, for example, with paints and coatings.